By DAVID M. HERSZENHORN

May 6, 2014

VIENNA — Arrested on bribery charges at the request of the F.B.I., and freed on more than $170 million bail that a Russian friend paid in cash, Dmitry V. Firtash, one of Ukraine’s richest, most powerful and most secretive businessmen, sat at a conference table here in his office at Centragas Holding, one of the many companies owned by his international conglomerate, Group DF.

With bodyguards posted in the hallway and his heavily armored Mercedes parked outside, Mr. Firtash is at once flamboyant and reclusive, insisting that the American authorities had no reason to arrest him and that he has nothing to hide, but refusing, as he has for years, to even estimate how much he is worth. Yet Mr. Firtash, who had close ties to the ousted Ukrainian president, Viktor F. Yanukovych, is not shy about the power he wields in Ukraine.

“Let me tell you something,” Mr. Firtash said in a recent four-hour interview, his eyes flashing intensely across a bowl of Austrian and Swiss chocolates. “When you say that business and politics are inseparable, that’s true. When you ask me the question, ‘Can I influence politics?’ I don’t know how to lie, so I’ll tell you: Yes, I can influence politics. I am not a politician, but I have certain influences.”

Mr. Firtash’s murky dealings with Gazprom, the Russian energy giant, have long added to American suspicions that he has inside information about how kickbacks from natural gas sales might flow to the highest levels of the Kremlin — details that could be critically useful as the United States tries to pressure President Vladimir V. Putin over Russia’s intervention in Ukraine.

The United States attorney’s office in Chicago, which led the investigation into Mr. Firtash, 48, flatly denied that the timing of his arrest by the Austrian authorities in March — on the same day that Ukraine’s new prime minister was visiting President Obama in the Oval Office — was linked to recent events in Ukraine, even though an indictment naming him and five others had been put under seal nearly a year earlier, in June 2013.

For the Americans, Mr. Firtash’s arrest, on charges of paying $18.5 million in bribes to Indian officials for a titanium mining deal, put Ukraine’s moguls on notice that acting against the West in the quickly escalating geopolitical fight with Russia would have serious repercussions. It also stirred hopes that Mr. Firtash could be pressured to pull back the curtain on corruption schemes that have crippled Ukraine politically and economically for years and spurred gas wars that left Europe freezing in winter.

But if the arrest was intended to send a message, Mr. Firtash quickly answered with his own, insisting that he is not the pro-Russian caricature of corruption that the Americans are making him out to be, and that he will not go down without a fight.

He has assembled a stable of top-flight lawyers, including Jamison Firestone, the onetime partner of Sergei L. Magnitsky, a Russian lawyer who died in prison after trying to expose a huge government tax fraud and whose name Congress immortalized on an American law seeking to punish Russian human rights abusers. Mr. Firtash has also hired Lanny J. Davis, the Washington crisis manager and former special counsel to President Bill Clinton, as well as Dan K. Webb, a former United States attorney for the Northern District of Illinois.

Shortly after his release from jail, Mr. Firtash helped set up talks at a luxury hotel here between two leading pro-Western Ukrainian presidential candidates: a fellow business titan, Petro Poroshenko, and a former champion boxer, Vitali Klitschko. The two men quickly reached a deal to unite behind Mr. Poroshenko, who is generally well-regarded in the West and now holds a commanding lead.

Helping unite the two candidates while under arrest, at a time when others might be in a defensive crouch or lying low, was the sort of brash stroke that has enabled Mr. Firtash to amass a fortune in the cutthroat realm of natural gas trading in the former Soviet sphere, and has earned him friends who will pay his bail in cash and enemies who have tried to ruin him.

It is also the kind of move for which he makes no apologies, like his longtime support of Mr. Yanukovych, with whom he shared a nemesis, former Prime Minister Yulia V. Tymoshenko, who is now running against Mr. Poroshenko.

Senior White House officials declined to discuss Mr. Firtash’s case. Antony J. Blinken, the deputy national security adviser, called it an “active legal matter.” State Department officials in the United States, Ukraine and Russia said they were prohibited from talking about it.

The potential value of information that Mr. Firtash might supply has soared as the dispute with Russia has deepened and officials at the Treasury Department aggressively seek to identify “cronies” of Mr. Putin who could be subject to economic sanctions by the West.

But to Mr. Firtash and his legal team, his arrest is yet another example of American overreach. He vociferously proclaimed his innocence in the office interview, and his lawyers also described the case against him as flimsy, largely built on a failed effort by the Federal Bureau of Investigation to pressure a former employee of a company involved in the titanium mining deal into false testimony. The lawyers also say American officials tried to block the transfer of funds sent by Vasily Anisimov, a Russian oligarch, to pay Mr. Firtash’s bail.

The former employee, Gajendra Lal, an Indian citizen and longtime resident of the United States, ultimately fled to Moscow to avoid the pressure of federal investigators. In an interview in Moscow, Mr. Lal accused American law enforcement officials of harassment and said they demanded that he lie to entrap Mr. Firtash. He has also made a video statement at the request of Mr. Firtash’s lawyers, which they plan to use to allege prosecutorial and F.B.I. misconduct.

Mr. Lal said he was told by prosecutors to go to Budapest to meet with associates of Mr. Firtash. “We were going through the script, and I said, ‘Look, some of this stuff is not true, absolutely untrue,’ ” Mr. Lal said in the video statement. He said one prosecutor told him: " ‘You’re on the side of the good guys now. You are allowed to lie and say whatever you want as long as you can get them to reply in this fashion, in this manner, to use these words.’ ”

“I felt I was being asked to lie and to lead people on,” he added, “Physically, I couldn’t do it.” Mr. Lal was also indicted in the bribery case. The F.B.I. declined to comment on Mr. Lal’s allegations.

Many of Mr. Firtash’s companies are the corporate version of Russian nesting dolls, shells within shells within shells, often registered in tax havens with obscure names. American investigations into Mr. Firtash and his businesses date back to at least 2005, when he created a company called RosUkrEnergo, which was a joint venture with Gazprom. Initially, Mr. Firtash tried to obscure his ownership stake, with his portion of the company held in a trust by an Austrian bank. Eventually, he admitted his role, though it remains unclear why he was trying to hide it.

Mr. Firtash came to prominence again after a classified State Department cable that was made public by WikiLeaks described him meeting with the American ambassador in Kiev, the Ukrainian capital, and acknowledging an association with a Russian mob figure, Semyon Y. Mogilevich, who is on the F.B.I.'s 10 most-wanted list.

The cable said Mr. Firtash admitted that he had to obtain Mr. Mogilevich’s permission to enter the gas-trading business. Mr. Firtash has denied the accuracy of most of the cable. Mr. Lal said in the interview that he would pass by Mr. Mogilevich’s wanted poster in the F.B.I. headquarters in Chicago, but that officials never wanted to discuss him.

“They were only interested in Dmitry,” Mr. Lal said.

Correction: May 8, 2014

Because of an editing error, an article on Wednesday about the Ukrainian businessman Dmitry V. Firtash, and how his connections to Gazprom, the Russian energy company, and to Ukrainian politicians make him a crucial figure in the Ukrainian crisis misidentified a former employer of Gajendra Lal, who figured in a criminal case against Mr. Firtash brought by the United States government. Mr. Lal worked for a company helping to set up a titanium mining deal in India; he did not work for the F.B.I.